Activated silica enriched mazzite, production method and application thereof

ABSTRACT

The invention provides an activated silica enriched mazzite having a chemical formula in the anhydrous state, expressed in molar ratios of: 
     a M 2/n  O; Al 2  O 3  ; b SiO 2  ; 
     with a variation from 0 to 0.5, M being an alkaline cation of valency n and b being greater than 7, and which has an acid strength corresponding to an amount of heat measured by ammonia absorption microcalorimetry, greater than 190 KJ/mol, and a pore volume, measured by cyclohexane adsorption, greater than 0.09 ml/g. The activated mazzites are useful in the conversion of hydrocarbons, in particular, the isomerization of C 4  to C 8  paraffin, and as molecular sieves in processes for the separation of hydrocarbons.

BACKGROUND OF THE INVENTION

1) Field of the Invention

The subject of the present invention is a silica-enriched activatedmazzite obtained by a crystallization gel containing zeolite crystals.The present invention also relates to its process of preparation and toits application as reaction catalyst for the conversion of hydrocarbons,in particular by isomerization, or as molecular sieves.

2) Background Art

Obtained for the first time by a synthetic route in 1966 under the nameof zeolite omega by Flanigen and Kellberg (U.S. Pat. No. 4,241,036),mazzite was identified in 1972 in basaltic rocks from Mont Semiol, nearMontbrison, Loire, France. Its crystalline structure was resolved byGalli (Cryst. Structure Comm., 3, 339, 1974) and Rinaldi et al. (ActaCryst., B31, 1603, 1974). From its structure, of hexagonal symmetry, itbelongs to the category of highly acidic zeolites with broad pores andunidirectional porosity, which makes it particularly advantageous forapplications in catalysis, in particular for the conversion ofhydrocarbons.

The omega claimed by Flanigen (U.S. Pat. No. 4,241,036) is characterizedby an X-ray spectrum common to all the types of mazzites preparedsubsequently, such as ZSM-4, LZ 202 or MZ-34. However, these mazzites,although identical in structure, are distinguished from one another bytheir specific synthetic route and different physical characteristics,such as the Si/Al ratio, their specific surface and their porosity.

In order to obtain a mazzite, so-called crystallization gels, containinga trivalent aluminium source, a silicon source, at least one alkalimetal or alkaline earth metal cation in the hydroxide form, water andoptionally an organic structuring agent, can be formed. However, fromthese constituents, by varying the stoichiometry of the gel or theconditions of the subsequent hydrothermal treatment necessary for thecrystallization, it is possible to obtain zeolites which are verydifferent from mazzite, such as offretite (OFF) or zeolite L (LTL) (S.Ernst and J. Weitkamp, Catalysis Today, 19, 1994, 27-60).

In order to improve the physical characteristics of these mazzites,silica gels, colloidal silicas, precipitated silicas, silicates orhydrolyzable silicic esters have been introduced into the synthesis orcrystallization gel, as silicon source, and aluminates, aluminahydroxides, alumina in the pure or commercial form or amorphousaluminosilicates have been introduced into the synthesis orcrystallization gel as trivalent aluminium sources. These aluminiumsources were subsequently replaced by natural or synthetic crystallinealuminosilicates, mainly natural clays. Such a substitution has made itpossible to obtain homogeneous growth of the crystals, due to the slowand even dissolution of these aluminosilicates in the synthesis medium(Dwyer, U.S. Pat. No. 4,091,007; Fajula, U.S. Pat. No. 4,891,200).

Other methods have been developed for synthesizing a novel mazzite fromseeds of mazzite or of another zeolite in the presence or in the absenceof organic structuring agent containing alkylammonium ions (Cannan, U.S.Pat. No. 4,840,779, Di Renzo, FR 2,651,221 and FR 2,698,862).

Thus, by varying the crystallization temperature to between 90° C. and150° C. and the content of sodium cations and of organic structuringagent, such as tetramethylammonium, choline or p-dioxane, in thecrystallization gels, it has been possible to obtain the various knowntypes of mazzite called omega, ZSM-4, LZ 202 or MZ-34 (cf W. M. Meierand D. H. Olson, "Atlas of Zeolite Structure Types", Third RevisedEdition, Butterworth, London 1992).

However, whatever the method of synthesis employed above, it has beenimpossible to synthesize a mazzite exhibiting an Si/Al ratio in theprecursors or alternatively crude synthetic products greater than 5,these ratios generally being between 2.5 and 5, which corresponds to amolar aluminium concentration varying between 0.166 and 0.285. Now, thecombination of a high aluminium content and of a unidirectional porositydoes not favour the application of such an unactivated mazzite incatalysis, because the desired optimum Si/Al ratio is frequently greaterthan 10 for such an application, which can only be obtained bydealumination during an activation phase.

Activation phase is usually understood to mean the combined individualstages carried out after the synthesis of a zeolite and which aretargeted at rendering it active in catalysis and in adsorption. Theseindividual stages, generally preceded by a stage of calcination of thecrude synthetic zeolite precursor, comprise an ion exchange, then ahydrothermal treatment and an acidic washing.

These treatments are known to induce profound textural and structuralmodifications in zeolites, and mazzites in particular, which affecttheir porosity and their acidity. Thus, the calcination stage, intendedto decompose the organic structuring agent occluded within the pores ofthe mazzite precursor, causes partial degradation of the structure andthe formation of amorphous residues which remain trapped in the poresand which obstruct the channels of the mazzite. The lower the Si/Alratio in the precursor, the greater the loss in crystallinity and thegreater the amount of amorphous residues. This limits the catalyticefficiency of the mazzite and the effectiveness of the subsequentactivation treatments. In addition, the hydrothermal treatment andacidic washing stages result in the formation of non-bridging bonds,creating silanol defects which decrease the strength and the number ofacid centres, reducing the long-distance order in the zeolite lattice.

The aim of the present invention is to avoid the problems encountered bymazzites with an excessively low Si/Al ratio, which, after activation,contain amorphous residues and lattice defects which reduce thecatalytic performances, and the present invention is targeted at theproduction of a silica-enriched activated mazzite with improvedproperties and with a limited number of silanol defects in the latticewhich exhibits a larger number of available acid sites and in particularan increased acid strength.

The subject of the present invention is a silica-enriched activatedmazzite with a chemical formula, in the anhydrous state, expressed asmolar ratio, of

    aM.sub.2/n O.Al.sub.2 O.sub.3.bSiO.sub.2

with a varying from 0 to 0.5, M denoting an alkaline cation of valency nand b being greater than 7, characterized in that it exhibits an acidstrength, expressed as amount of heat of adsorption of ammonia, greaterthan 190 kJ/mol and a pore volume, measured by adsorption ofcyclohexane, greater than 0.09 ml/g.

According to the present invention, the acid strength of the activatedmazzite corresponds to the amount of initial heat of adsorption ofammonia; it is measured by microcalorimetry. The measurement consists inadsorbing gaseous ammonia on the activated mazzite at 150° C. and inmeasuring the amount of heat released. The activated mazzite accordingto the invention exhibits a particularly significant acid strengthbecause, until now, whatever the method of synthesis known and employedby a person skilled in the art, it has never been possible to obtain ashigh a value of the amount of heat of adsorption of ammonia on anactivated mazzite.

These measurements of acidity of the mazzite from the amounts of heat ofabsorption of ammonia are fully described in the article entitled "Amultitechnique characterization of the acidity of dealuminated mazzite"by D. McQueen, B. H. Chiche, F. Fajula, A. Auroux, C. Guimon, F.Fitoussi and Ph. Schulz, J. Catal., 1996, 161.

In order to obtain such an acid strength, the activated mazziteaccording to the invention is obtained from a zeolite precursor itselfsynthesized from a crystallization gel containing faujasite X, with anSi/Al ratio of less than 1.5, the said precursor subsequently beingsubjected to an activation treatment.

The Applicant Company had already synthesized mazzite from grains offaujasite Y with an Si/Al ratio of greater than 1.5, as is described inPatent FR 2,698,862. However, in contrast to what it had recommended,the molar composition of the crystallization gel is completelydifferent; in fact, the molar ratios vary

from 5 to 15 for SiO₂ /Al₂ O₃

from 1 to 2 for SiO₂ /TMA₂ O

from 0.03 to 0.25 for TMA₂ O/Na₂ O

and from 30 to 150 for H₂ O/Na₂ O

with TMA₂ O, the organic structuring agent, chosen fromtetraalkylammonium ions, each alkyl group comprising from 1 to 4 carbonsand preferably denoting the tetramethylammonium ion. The sodium isintroduced partly by the addition of sodium hydroxide used to adjust thealkalinity of the gel and the tetravalent silicon and trivalentaluminium sources originate partly from the group composed of silicates,solid or colloidal silicas, gels and xerogels, hydrolyzable silicicesters and diatomites and, on the other hand, from faujasite X. In thegel, it is possible, however, to have therein a mixture of alkalineions, it being possible for the latter in particular to be introduced byfaujasite X.

The use of commercially available faujasite X has made it possible toreduce both the cost of the starting materials and the cost ofmanufacture of the zeolite of mazzite type. This is because it is nolonger necessary to form grains with a specific configuration which aredelimited solely by rounded surfaces, the synthesis of which isexpensive, in particular in time (10 to 12 days of crystallization at50° C., with or without stirring), as was described in Patent FR2,698,862.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the process according to the invention, it is possible for theactivation treatment, comprising at least two stages, a first ionexchange stage and a second dealumination stage, to be carried outeither on the crude synthetic zeolite precursor after crystallization oron the precursor which has been subjected to the conventionalcalcination heat treatment in order to remove the organic structuringagent. This stage of calcination of the crude synthetic precursorconsists in heating under a flow of nitrogen or of air or of a mixtureof both for 5 minutes to 10 hours, preferably 5 hours, at a temperaturegreater than 450° C., preferably between 500 and 550° C. However, in apreferred form of the invention, the activation treatment is applied tothe crude synthetic precursor.

By dispensing with the calcination stage accepted until now as necessaryin order to remove the organic structuring agent, it has been possibleto obtain mazzites with crystalline lattices exhibiting fewer structuraldefects (detectable by ²⁷ Al NMR, infrared, and the like or othermethods known to a person skilled in the art), indeed no structuraldefects.

In a preferred form of the invention, the ion exchange stage consists inexchanging the alkaline ions of the crude synthetic zeolite precursorcontaining the organic structuring agent by washing by means of anaqueous solution of an ammonium salt and/or of an inorganic or organicacid with a pH of less than or equal to 3.

During the dealumination stage, the solid recovered after ion exchangeis maintained at a temperature of between 450 and 850° C. under a moistatmosphere for a period of time of 1 minute to 4 hours and then washedby means of an acidic buffer solution with a pH varying from 1 to 5, ata temperature varying from 20 to 100° C. The solid thus heat-treated iswashed during the second dealumination phase with a buffer solutioncomposed of a mixture of at least one ammonium salt and of at least oneinorganic or organic acid with a pH preferably of between 2 and 3. In apreferred form, the buffer solution will contain inorganic ammoniumsalts and their corresponding acids.

A second embodiment of the invention is the process for producingactivated mazzite comprising a stage of preparation of the mazziteprecursor and a stage of activation of this said mazzite precursor. Itis distinguished from the other known processes for obtaining activatedmazzite in that, in order to prepare the precursor, a faujasite X isintroduced into the composition of the crystallization gel, that theactivation treatment is applied to the precursor containing the organicstructuring agent and that the acidic dealumination washing is carriedout by means of a buffer solution of an acid salt and of an inorganic ororganic acid with a pH of less than 3, preferably of between 2 and 3.

In addition, the said process according to the invention is alsocharacterized by a molar composition of silica, alumina, alkaline ionsand organic structuring agent in the crystallization gel such that themolar ratios vary

from 5 to 15 for SiO₂ /Al₂ O₃

from 1 to 2 for SiO₂ /TMA₂ O

from 0.03 to 0.25 for TMA₂ O/Na₂ O

and from 30 to 150 for H₂ O/Na₂ O

with TMA₂ O, the organic structuring agent, chosen fromtetraalkylammonium ions, each alkyl group comprising from 1 to 4carbons, preferably tetramethylammonium ions, and the tetravalentsilicon and trivalent aluminium sources originating partly from thegroup composed of silicates, solid or colloidal silicas, gels andxerogels, hydrolysable silicic esters and diatomites and, on the otherhand, from faujasite X.

Compared with the conventional processes, the process according to theinvention exhibits a treatment which is simplified in time and intreatment cost by the use of readily available commercial zeolites andby the elimination of calcination stages which generate defects in theactivated mazzite.

Such activated mazzites according to the invention are particularlyefficient in the conversion of hydrocarbons, in particular theisomerization of C₄ to C₈ paraffins and in any conversion ofhydrocarbons which involves a bifunctional or acid catalysis mechanism,in particular as molecular sieves in processes for the separation ofhydrocarbons.

The examples given hereinbelow are targeted at clarifying the inventionbut may not, under any circumstances, limit the scope thereof.

EXAMPLE I

The present example is targeted at showing the superiority of themazzite according to the invention compared with the activated mazzitesaccording to the prior art as regards the acid strength.

Four samples of activated mazzite were prepared:

The activated mazzite characteristic of the invention, known as MAZ-X₀

the activated mazzite according to Patent FR 2,698,862, known as MAZ-Y₁

the activated mazzite obtained according to Patent FR 2,651,221, knownas MAZ-G

the activated mazzite obtained according to the procedure of U.S. Pat.No. 5,139,761, known as MAZ-T

The conditions for the preparation of these various zeolites are givenin Table I hereinbelow.

                                      TABLE I                                     __________________________________________________________________________           MAZ-X.sub.0                                                                          MAZ-Y.sub.1                                                                           MAZ-G   MAZ-T                                           __________________________________________________________________________    Composition of                                                                  the gel (in g)                                                                H.sub.2 O              9511.6         1004.8           50                                                 15,604                                            NaOH              752.8          66.1             2.62            6804                                     KOH              0 0                1.92 0       TMACl             130.8          148.2            0.97            2690                                    (TMABr)                                           SiO.sub.2 source       1663.6         149.6            8.10                                               57780                                              (Zeosil Z175)  (Cecagel, Ceca)  (Cegagel, Ceca) (Sodium silicate:                                             21.9% SiO.sub.2)                             Al.sub.2 O.sub.3 source      654            87               1.85                                              42770                                         (Siliporite    (Specific        (Specific       Alum (8.3% Al.sub.2                                      O.sub.3 +                                          Na)            Faujasite Y)     Faujasite Y)   24.1% H.sub.2 SO.sub.4                                    + 67.6%                                                                          H.sub.2 O)                                     Crystallization                                                               conditions                                                                    T° C.               115° C.          115° C.                                              115° C.           125°                                     C.                                                Pressure          Autogenous     Autogenous       Autogenous                                              Autogenous                                        Time             45 h         45 h           24 h             45 h                                         Drying           110° C.                                              110° C.          110° C.                                              100° C.                              (Time h)         (12 h)        (12 h)          (12 h)                         Calcination      No            550° C. (4 h)    550° C.                                     (4 h)     540° C. (1.5 h)                                                        under air      under air              __________________________________________________________________________

In all cases, the mazzite precursor is prepared under the conditionslaid down in Table I according to the order described hereinbelow.

In order to form the crystallization or synthesis gel for the MAZ-X₀,MAZ-Y₁ and MAZ-G, the sodium hydroxide and then the tetramethylammoniumchloride (TMACl) are dissolved in deionized water, the solution beingstirred mechanically at approximately 150 revolutions per minute, andthen the silica source is added gradually while continuing to stir over1 hour. Finally, the aluminium source is added, still while stirring thesolution and while maintaining it for a further two hours, this latterstirring phase corresponding to the maturing of the gel. The gel, thusmatured, is transferred to a reactor where it is kept stirring underautogenous pressure for 24 hours at 115° C. to enable the mazzitecrystals to grow. The solid collected or zeolite precursor is dried for12 hours in an oven at 115° C. After drying, the crystals are calcinedunder air at 550° C. for 4 hours. For the mazzite according to theinvention, this calcination stage has been eliminated. To prepare theMAZ-T, the procedure described in Examples 1 and 2 of U.S. Pat. No.5,139,761 has been followed.

The specific conditions of the activation treatment for each of themazzites sampled is given in Table II hereinbelow.

                                      TABLE II                                    __________________________________________________________________________            MAZ-X.sub.0                                                                             MAZ-Y.sub.1                                                                          MAZ-G  MAZ-T                                         __________________________________________________________________________    Ion exchange                                                                    Solution         (NH.sub.4).sub.2 SO.sub.4          NH.sub.4 NO.sub.3                                             NH.sub.4 NO.sub.3        NH.sub.4                                       NO.sub.3                                        (Concentration)  (0.5 M)              (0.5 M)         (0.5 M)                                               (0.5 M)                                         T° C.              80° C.                80° C.                                               80° C.          93°                                      C.                                              Time            1 h                 1 h            1 h           1 h                                         Washing          yes                 yes                                                yes           yes                    Drying                                                                        Dealumination                                                                 Heat treatment                                                                100% H.sub.2 O                                                                T° C. (Time in h) 620° C. (2 h)         620° C. (2                                     h)    620° C. (2 h)   550°                                      C. (2 h)                                        Acidic washing   Buffer             HNO.sub.3 (1.5 N)    HNO.sub.3 (1.5                                     N)  NH.sub.4 NO.sub.3 + HNO.sub.3                                               (NH.sub.4).sub.2 SO.sub.4 (0.5 M) +                                           HNO.sub.3 (1 N)                               T° C.              reflux              reflux         reflux                                            93° C.                                Crystallinity    95%                 90%            80%                     __________________________________________________________________________                                    85%                                       

This activation treatment comprises a first ion exchange stage and asecond dealumination stage comprising two phases, the first of treatmentwith steam and the second of acidic washing.

The stage of ion exchange of the sodium ions is carried out by washingthe calcined or non-calcined (invention) zeolite precursor with anammonium salt solution with compositions described in Table II at atemperature of less than 100° C. The treatment with steam consists inleaving the zeolite in an atmosphere of steam at a temperature ofgreater than 500° C. for at least 2 hours. The acidic washing stageconsists in washing the solids obtained with an acidic solution with apH and composition given in Table II for at least half an hour, suchthat the ratio of the volume of solution to the weight of zeolitetreated is less than or equal to 10 ml/g.

The crystallinities of the activated mazzites, measured by X-raydiffraction, are given in Table II and their respective acid strengths,expressed in kilojoules per mol, are given in Table III.

The acid strength of each of these activated mazzites is measured byusing the ammonia adsorption microcalorimetry technique described by A.Auroux in "Catalyst, Characterization, Fundamental and AppliedCatalysis. Physical Techniques for Solid Materials", edited by B. Imelikand J. C. Vedrine, Plenum Press, New York, 1994, which makes it possibleto study the acid strength of the sites present, the strongest acidsites being measured by measuring the initial heat of desorption of NH₃.

                  TABLE III                                                       ______________________________________                                               MAZ-X.sub.0                                                                          MAZ-Y.sub.1 MAZ-G   MAZ-T                                       ______________________________________                                        Q ads    205      190         165   170                                         (kJ/mol)                                                                      Acidity    0.385      0.250      0.190       0.205                            in meq                                                                        H.sup.+ /g                                                                  ______________________________________                                    

It is found that the acid strength and the number of acid sites are muchgreater for the activated mazzite according to the invention.

EXAMPLE II

The present example is targeted at showing the specificity of theprocess for producing activated mazzite according to the invention fromfaujasite X, compared with the process for the preparation of activatedmazzite obtained from faujasite Y.

Several samples were tested:

the first is the activated mazzite MAZ-X₀, in accordance with theinvention, prepared as described in Example I,

the second is the activated mazzite obtained from faujasite Y(especially synthesized as in U.S. Pat. No. 2,698,862) under theconditions of the present invention, hereinafter known as (MAZ-Y₀)_(a),

the third is an activated mazzite obtained like (MAZ-Y₀)_(a) butobtained from a commercial faujasite Y, hereinafter known as(MAZ-Y₀)_(b),

the fourth is an activated mazzite obtained from faujasite X under theconditions of Patent FR 2,698,862, hereinafter known as MAZ-X₁,

the fifth is the activated mazzite MAZ-Y₁ in accordance with theinvention of Patent FR 2,698,862 and prepared as described in Example I.

The characteristics of the crystallization gels and of the crystallinityobtained for each sample after the first stage of synthesis and afteractivation are collated in Table IV hereinbelow.

                                      TABLE IV                                    __________________________________________________________________________           MAZ-X.sub.0                                                                          (MAZ-Y.sub.0).sub.a                                                                  (MAZ-Y.sub.0).sub.b                                                                  MAZ-X.sub.1                                                                         MAZ-Y.sub.1                                 __________________________________________________________________________    Crystallization                                                                 gel (g)                                                                       H.sub.2 O                9511.6           9510        9510         1004                                               1004.9                                NaOH               752.8            753         753          67                                                 67                                          TMACl              130.8            130         130          149                                                149                                         SiO.sub.2 source        1669.6 (a)      1664 (a)    1663 (a)     150                                          (b)     149.6 (b)                             Al.sub.2 O.sub.3 source       654.4 (c)       655 (d)     656 (e)                                             88 (c)      87.6 (d)                          Morphology of      Hexagonal        amorphous   amorphous   Essentially                                        Hexagonal                                    the crystals and   prisms of       % MAZ < 25% % MAZ < 25   amorphous                                         prisms of                                     crystallinity      2 × 1 μm                                                                           product      2.5 × 1.5 μm                                             approximately                                                                                    100%                     100%                                                                        __________________________________________________________________________

(a)=Zeosil Z145, sold by Rhone-Poulenc.

(b)=Cecagel, sold by Ceca

(c)=Siliporite NaX, sold by Ceca

(d)=NaY, obtained according to the synthetic process described in FR2,698,862

(e)=NaY, sold by Zeocat (ZF 110 with a ratio Si/Al=2.7).

It is found that the preparation of mazzite "Y" from faujasite Yaccording to the present invention is impossible, whatever the size orthe shape of the crystals of the said faujasite Y. Likewise, mazzite "X"cannot be prepared from faujasite X according to the procedure of PatentFR 2,698,682. The two routes for producing mazzite via the faujasites Xand Y are entirely distinct.

EXAMPLE III

The present example describes the use of the activated mazzite accordingto the invention in the composition of catalysts for the isomerizationof paraffins (in this instance, n-hexane), compared with the use ofother activated mazzites of the prior art in these catalysts.

In order to prepare these isomerization catalysts, each sample ofactivated mazzite is kneaded with alumina and water in order to form apasty mixture containing 20% by weight of alumina. The mixture isextruded through a die; the dried and calcined extrudates exhibit adiameter of 1.6 mm and a length varying from 3 to 5 mm.

0.3% by weight of platinum is deposited on these extrudates,constituting the support of the catalyst, by cation exchange of thesupport with the salt Pt(NH₃)₄ Cl₂.H₂ O in the presence of competitorions, for example ammonium nitrate.

The catalyst is subsequently calcined under air at 520° C. and then themetal is reduced under hydrogen flow, the temperature being graduallyraised from 150 to 450° C. At the end of the treatment, the metal phasesare completely divided and distributed within the solid.

The comparative tests of activity of the catalysts thus formed frommazzites are carried out in a stationary-bed catalytic unit with theoperating conditions hereinbelow:

temperature of the reactor between 150 and 300° C.,

pressure of 1 atmosphere,

a hydrogen/n-hexane molar ratio equal to 70,

an HVR (space velocity), expressed by the ratio of the masses of chargeand of catalyst, equal to 0.2 h⁻¹.

The performances of these catalysts are defined from the followingcharacteristics:

T_(50%) =temperature necessary in order to obtain a conversion of then-hexane of 50%.

Conversion is understood to mean the percentage of product isomerizedwith respect to the amount of the same product present in the charge.

Y_(i) =yield of dibranched isomers at 50% conversion of the n-hexane,with i=23DMB corresponding to the 2,3-dimethylbutane isomer and i=22DMBcorresponding to the 2,2-dimethylbutane isomer. This yield correspondsto 100 times the ratio of the mass of isomer under consideration to thetotal mass of the hydrocarbons after isomerization.

T_(crack) =temperature of appearance of the cracking products in °C.

Conv.=optimum conversion in % which can be achieved with a catalystunder consideration.

T_(conv). =temperature at which the optimum conversion is achieved in°C.

The results are collated in Table V hereinbelow.

                  TABLE V                                                         ______________________________________                                        Catalys          Y.sub.22DMB                                                                            Y.sub.23DMB                                                                         T.sub.crack                                                                        Conv. T.sub.conv.                          based on:      T.sub.50% (° C.)            (%)          (%)                                                          (° C.)                                                            (%)          (°             ______________________________________                                                                                   C.)                                MAZ-X.sub.0                                                                          182       5.8      8.1   226  90    232                                  MAZ-Y.sub.1       190          5.2          8.0           232                                                           85           245                    MAZ-G        200          4.9          7.3           231            86                                                          255                         MAZ-T        205          4.6          7.4           237            82                                                          260                       ______________________________________                                    

It is found, from this table, that the catalyst prepared from MAZ-X₀ inaccordance with the invention is more active (lower T_(50%) andT_(conv). working temperatures to obtain a greater degree of conversion)and more selective for dibranched isomers than the other catalysts ofthe prior art.

We claim:
 1. Silica-enriched activated mazzite with a chemical formula,in the anhydrous state, expressed as molar ratios, of

    aM.sub.2/n O.Al.sub.2 O.sub.3.bSiO.sub.2

with a varying from 0 to 0.5, M being an alkaline cation of valency nand b being greater than 7, which exhibits an acid strength,corresponding to the amount of heat measured by ammonia adsorptionmicrocalorimetry, greater than 190 kJ/mol and a pore volume, measured byadsorption of cyclohexane, greater than 0.09 ml/g.
 2. Activated mazziteaccording to claim 1, which is obtained from a zeolite precursor itselfsynthesized from a crystallization gel containing faujasite X, the saidprecursor being subjected to an activation treatment.
 3. Activatedmazzite according to claim 2, wherein the crystallization gel exhibits amolar composition of silica, alumina, alkaline ions and organicstructuring agent such that the molar ratios varyfrom 5 to 15 for SiO₂/Al₂ O₃ from 1 to 2 for SiO₂ /TMA₂ O from 0.03 to 0.25 for TMA₂ O/Na₂ Oand from 30 to 150 for H₂ O/Na₂ Owith TMA₂ O, the organic structuringagent, chosen from tetraalkylammonium ions, each alkyl group comprisingfrom 1 to 4 carbons, and the tetravalent silicon and trivalent aluminiumsources originating partly from the group composed of silicates, solidor colloidal silicas, gels and xerogels, hydrolysable silicic esters anddiatomites and, optionally, from faujasite X.
 4. Activated mazziteaccording to claim 2, wherein the zeolite precursor, with or withoutorganic structuring agent, is activated in at least two stages, a firstion exchange stage and a second dealumination stage.
 5. Activatedmazzite according to claim 4, wherein, during the ion exchange stage,the alkaline ions of the zeolite precursor containing the organicstructuring agent are exchanged by washing by means of an aqueoussolution of an ammonium salt and/or of an inorganic or organic acid witha pH of less than or equal to
 3. 6. Activated mazzite according to claim4, wherein, during the first phase of the dealumination stage, the solidobtained after ion exchange is maintained at a temperature of between450 and 850° C. under a moist atmosphere for from 1 minute to 4 hoursand then washed by means of an acidic buffer solution with a pH varyingfrom 1 to 5, at a temperature varying from 20 to 100° C.
 7. Activatedmazzite according to claim 4, wherein, during the second dealuminationphase, the heat-treated solid is washed with a buffer solution composedof a mixture of at least one inorganic or organic acid with at least oneammonium salt of at least one inorganic or organic acid with a pH ofbetween 2 and
 3. 8. Activated mazzite according to claim 7, wherein thebuffer solution is a mixture of an ammonium salt and of the acidcorresponding to this said salt.
 9. Process for producing the activatedmazzite according to claim 1 comprising a stage of preparation of thezeolite precursor and a stage of activation of the said zeoliteprecursor, wherein the crystallization gel contains a faujasite X, thatthe activation treatment is applied to the zeolite precursor containingthe organic structuring agent and that an acidic dealumination washingis carried out by means of a buffer solution of an acid salt and of aninorganic or organic acid with a pH of less than
 3. 10. Processaccording to claim 9, wherein the crystallization gel exhibits a molarcomposition of silica, alumina, alkaline ions and organic structuringagent such that the molar ratios varyfrom 5 to 15 for SiO₂ /Al₂ O₃ from1 to 2 for SiO₂ /TMA₂ O from 0.03 to 0.25 for TMA₂ O/Na₂ O and from 30to 150 for H₂ O/alkaline ionwith TMA₂ O, the organic structuring agent,chosen from tetraalkylammonium ions, each alkyl group comprising from 1to 4 carbons, and the tetravalent silicon and trivalent aluminiumsources originating partly from the group composed of silicates, solidor colloidal silicas, gels and xerogels, hydrolysable silicic esters anddiatomites and, optionally, from faujasite X.
 11. The activated mazziteaccording to claim 3, wherein the tetralkylammonium ions aretetramethylammonium ions.
 12. The process according to claim 10 whereinthe tetralkylammonium ions are tetramethylammonium ions.
 13. In aprocess for the isomerization of C₄ to C₈ parafinic hydrocarbons, theimprovement which comprises conducting the isomerization in the presenceof the activated mazzite of claim
 1. 14. In a process for the conversionof hydrocarbons which involves a bifunctional or acid catalyticmechanism, the improvement which comprises conducting the conversion inthe presence of the activated mazzite of claim 1.